Drives and dmg's are mounting okay on my Mac, and I can find them at /Volumes/. However, they aren't appearing in the sidebar in the Finder. What's the best way to start fixing this?
In Finder you can show or hide the disks and other devices
Go to the sidebar and mouse over DEVICES you get to the right text that says Show or Hide. Click on that word to toggle the display of the devices.
I faced a similar problem today, but the "devices" show/hide solution didn't quite solve it for me.
However, going to the Finder's Preferences > Sidebar > Devices, the box next to "External disks" was neither empty (unselected) nor check-marked (selected), but rather marked with a hyphen (partially selected). After clicking the box next to "External disks" until the check-mark appeared, my mounted image started to show up in Finder.